The invention relates to a twist-grip shift mechanism for bicycles and in particular a twist-grip shift mechanism having a housing part formed by an injection molding process using a single half of an injection mold to eliminate undercuts.
German Patent Application DE 197 23 346.5 discloses a shift mechanism for controlling transmissions on bicycles, in particular a twist-grip shift mechanism to be fitted on the handlebars. This shift mechanism includes a fixed housing connected to the handlebars and a housing that is fixed axially with the fixed housing and can be rotated by hand. The fixed housing and the rotatable housing form a unit. The rotatable housing has an inner cylinder with outward-facing recesses and the fixed housing has an outer cylinder with bayonet hooks on its axially outer end face. The inner cylinder of the rotatable housing may be pushed onto the outer cylinder of the fixed housing to form a bearing. The two housings may be connected permanently to one another by rotating the rotatable housing relative to the fixed housing until a latching point is reached.
A problem associated with the above configuration is the proper molding of the two housing to form a satisfactory bearing. During the molding process, undercuts may form. The undercuts cause parting seams and molding inaccuracies which may cause connecting problems when the two housings are bought together to form the bearing. Therefore, there is a need for a shift mechanism that eliminates the above stated problem.
The present invention provides a twist-grip shift mechanism for bicycles with a fixed housing which has a cylindrical extension surrounding a handlebar to support a rotating part. To eliminate the above stated problems the cylindrical extension is configured in such a way that no contours that connect with the rotating part go beyond the outside diameter of the cylindrical extension. Another factor that contributes to an efficient manufacturing process is that there is a housing extension surrounding a cable bobbin on the rotating part. The housing extension is formed as part of the housing part. The contours are formed on the outside diameter of the cylindrical extension during an injection molding method using a single mold half of an injection mold. The advantage of this manufacturing method is a homogeneous cylindrical surface of the cylindrical extension, which serves as a bearing for the rotating part resulting in no parting seams or molding inaccuracies that could impair operation of the twist-grip shift mechanism during actuation.
The above-mentioned manufacturing method may also be used for molding tongues with retaining profiles onto the cylindrical extension for releasable assembly with latching profiles of a ring. The retaining profiles include apertures to prevent them from projecting beyond the limits determined by the inner and outer diameters of the cylindrical extension. The latching profiles on the ring are designed as hooks engageable with the apertures in the tongues.
The rotating part and the cable bobbin may be encapsulated with a grip part made of rubber-elastic material. Such a configuration allows a radially inward-pointing lip to be molded on the grip that extends beyond the overall length of the rotating part on the side opposite the cable bobbin. In the installed condition, the lip engages in a radially outward-facing groove arranged in the ring. This allows the bearing between the rotating part and the cylindrical extension to be kept free from penetrating dirt, thus ensuring the functioning of the lubricated bearing over a prolonged period.
It is therefore the object of the invention to provide an injected molded housing part for a twist-grip shift mechanism having a cylindrical extension that includes an outer functional surface and a housing extension that includes inner contours that is configured to allow the use of a single half of an injection mold, thereby eliminating undercuts and other molding inaccuracies.